1. Field of the Invention
This invention relates to gaskets, and more particularly to gaskets for use in joints for ultra-high vacuum devices or devices requiring hermetic seals.
2. Description of the Prior Art
Ultra-high vacuum environments, and devices capable of maintaining an ultra-high vacuum are essential in a number of scientific and commercial settings including surface science studies, space simulation chambers and high energy physics research.
Similarly, hermetic seals are required in joints between members which separate very high and low pressure environments. For work at pressures less than 10.sup.-8 torr, it is necessary to bake the entire chamber of an ultra-high vacuum device and all its flanges to several hundred degrees Celsius in order to outgas all of the internal surfaces. Vacuum systems of this type thus require near perfect seals which are able to withstand the temperatures and pressure demands of the system and are also able to maintain their seals over a wide range of temperatures and pressures at all aperture points and openings to the ambient environment outside the vacuum system.
Temperature and pressure demands of systems under positive pressure, such as high pressure pipes, also require tight seals. Other services requiring near-perfect seals include pressurized chambers and cryogenic systems.
Especially important are the seals that must be provided for inlets through the chamber wall of an ultra-high vacuum or high pressure system and which allow for the performance of specialized functions within the vacuum chamber. For instance, in order to provide inlets for electricity, refrigerants, motion, gas or light into a vacuum or pressure system, hermetically sealed feedthroughs are required. Presently, these feedthroughs are generally welded or brazed to a flange or to the chamber wall of a vacuum chamber. Other flanges are sold with set configurations of permanently mounted electrical feedthroughs. If it is desired that a feedthrough be demountable, it is usually permanently mounted to a demountable flange. The flange is then sealed to a mating flange on the vacuum chamber using a captured elastomeric O-ring or soft metal ring gasket.
An alternative method of incorporating a feedthrough into a vacuum or pressure system is by the use of feedthroughs mounted on threaded fittings. The male fitting is sealed to the female thread which has been formed in a flange or through the chamber wall using Teflon tape as a gasket material to effect a better seal. The gasket materials used in vacuum systems are chosen on the basis of their outgassing rates, chemical inertness, thermal stability, and their ability to flow to fill microscopic defects on the two sealing surfaces. Concurrently, threaded fittings are not commonly used for ultra-high vacuum systems, as the integrity of the Teflon seal is destroyed at temperatures above 200 degrees Celsius. Bakeable ultra-high vacuum systems usually employ all metal (copper) gasket seals which will withstand baking up to about 450 degrees Celsius.
Pipe thread gaskets of metal have been proposed in the past for use in pipes carrying fluids under pressure. These gaskets are threaded and intended for inclusion in pipes having male and female ends that are of tapered tubular structure. However, the known types of pipe thread gaskets operate on a principle whereby a complete sealing of each thread in a pipe threaded joint is required, a capability that in practice is difficult to achieve. Further, such gaskets require large amounts of torque to be applied as the members are being threaded together. Heretofore lacking in the prior art is a metal gasket that will permit the use of commercially available feedthroughs on threaded fittings in ultra-high vacuum or in pressurized systems, particularly in systems which experience high temperature cycling, such as the cycling experienced in or baking and degassing processes, or in cryogenic service.